How to synchronize the brightness of a grille-mounted neon tube/LED bar to a car's tach/speedo?

I have a 1997 BMW 328i, and I've been thinking about installing either a neon tube or a bar of LED's in the grille of the car to use at night (most likely not street legal, I know). I want the brightness of the tube/bar to be controlled by one of two things: The speedometer or the tachometer. I.e., if the speedometer is at 0 MPH, the light is off, and if the speedo is at 75 MPH, the light is fully on. Same thing with the tach, light off at 0 RPM, light fully on at redline. Unfortunately, I haven't the slightest idea how to use the car's computer or the gauges themselves to set up a control module for the light. If any of you has a good amount of knowledge about automotive gauges, I would appreciate your help!

Marsh8 years ago
You could use an LM3915 chip to drive a bar of LEDs.
The issue is going to be getting the signal from the engine. In the olden days you could just tap the points from the distributor to get a pulse to drive a counter. Nowadays, cars don't have points and a lot of them don't even have distributors anymore so getting that signal is going to be problematic.
I know this is not a complete solution, but hang on to it anyway because I think this is a piece you'll be able to use.
Better to use an LM3914, the output is linear, the 3915 is logarithmic. I'd tap the alternator for a speed feed.